Polyester polyols are commonly used intermediates for the manufacture of condensation polymers. These condensation polymers include polyurethane products such as flexible and rigid polymeric foams, polyisocyanurate foams, coatings, powder coatings, sealants, adhesives, and elastomers.
Commonly, the polyester polyol is made by condensing aromatic diacids, diesters, or anhydrides (e.g., terephthalic acid, dimethyl terephthalate) with glycols such as ethylene glycol, propylene glycol, diethylene glycol, or the like. These starting materials usually derive exclusively from petrochemical sources.
As companies increasingly seek to offer products with improved sustainability, the availability of intermediates produced from bio-renewable and/or recycled materials becomes more leveraging. However, there remains a need for these products to deliver equal or better performance than their traditional petroleum-based alternatives at a comparable price point.
Bio-renewable content alone can be misleading as an indicator of “green” chemistry. For example, when a food source such as corn is needed to provide the bio-renewable content, there are clear trade-offs between feeding people and providing them with performance-based chemical products. Additionally, the chemical or biochemical transformations needed to convert sugars or other bio-friendly feeds to useful chemical intermediates such as polyols can consume more natural resources and energy, and can release more greenhouse gases and pollutants into the environment than their petro-based alternatives in the effort to achieve “green” status.
Waste thermoplastic polyesters, including waste polyethylene terephthalate (PET) streams (e.g., from plastic beverage containers), provide an abundant source of raw material for making new polymers. Usually, when PET is recycled, it is used to make new PET beverage bottles, PET fiber, or it is chemically transformed to produce polybutylene terephthalate (PBT). Other recycled raw materials are also available. For example, recycled propylene glycol is available from aircraft or RV deicing and other operations, and recycled ethylene glycol is available from spent vehicle coolants.
Urethane formulators demand polyols that meet required specifications for color, clarity, hydroxyl number, functionality, acid number, viscosity, and other properties. These specifications will vary and depend on the type of urethane application. For instance, rigid foams generally require polyols with higher hydroxyl numbers than the polyols used to make flexible foams.
Polyols suitable for use in making high-quality polyurethanes have proven difficult to manufacture from recycled materials, including recycled polyethylene terephthalate (rPET). Many references describe digestion of rPET with glycols (also called “glycolysis”), usually in the presence of a catalyst such as zinc or titanium. Digestion converts the polymer to a mixture of glycols and low-molecular-weight PET oligomers. Although such mixtures have desirably low viscosities, they often have high hydroxyl numbers or high levels of free glycols. Frequently, the target product is a purified bis(hydroxyalkyl) terephthalate (see, e.g., U.S. Pat. Nos. 6,630,601, 6,642,350, and 7,192,988) or terephthalic acid (see, e.g., U.S. Pat. No. 5,502,247). Some of the efforts to use glycolysis product mixtures for urethane manufacture are described in a review article by D. Paszun and T. Spychaj (Ind. Eng. Chem. Res. 36 (1997) 1373.
Most frequently, ethylene glycol is used as the glycol reactant for glycolysis. This is sensible because it minimizes the possible reaction products. Usually, the glycolysis is performed under conditions effective to generate bis(hydroxyethyl) terephthalate (“BHET”), although sometimes the goal is to recover pure terephthalic acid. When ethylene glycol is used as a reactant, the glycolysis product is typically a crystalline or waxy solid at room temperature. Such materials are less than ideal for use as polyol intermediates because they must be processed at elevated temperatures. Polyols are desirably free-flowing liquids at or close to room temperature.
The safe disposal or reuse of waste materials from various sources is an environmental and economic challenge. Such wastes had typically gone into landfills, but as landfill capacity is becoming ever scarcer and disposal costs are continuously increasing, cost effective and environmentally acceptable alternatives are needed to deal with these waste materials. Waste streams are produced by a great range of industries and sources, including, e.g., the plastics industry, the automobile industry, the paper industry, consumers, the agricultural industry, including both crop and animal production, as well as the production of animal products (e.g., the dairy, egg, and wool industries). Because of these environmental and cost challenges, there is a need to find practical uses for recycled polymers and waste streams. In other words, there is the need to utilize recycled polymers and waste streams to produce new polymers and building blocks for these new polymers.
Chemolysis, which is the chemical breakdown or decomposition of an organic molecule into smaller molecules, may provide a route for recycling of polymeric materials. Chemolysis is essentially a depolymerization process and can be viewed as the opposite of a polycondensation process to make a polymer. Chemolysis is usually applied to condensation polymers such as PET, polyurethanes, or polyamides. However, chemolysis is not applicable to polymers such as vinyls, acrylics, fluoroplastics and polyolefins, and by some estimates not applicable to more than about 10% of plastics waste. See, e.g., “Survey of current projects for plastics recycling by chemolysis”, European Commission Joint Research Centre, Institute for Prospective Technological Studies, Seville, May 1996. Therefore, the recycling or chemolysis of many recycle polymers and waste streams presents significant technical challenges.
In many instances, it would be highly desirable to have improved polyester polyols. In particular, the urethane industry needs sustainable polyester polyols based in substantial part on recycled polymers or waste streams. This scenario would provide a viable means for consuming these recycle waste streams. Furthermore, polyester polyols with high recycle content that satisfy the demanding viscosity, functionality, hydroxyl content, and performance requirements of formulators, such as polyurethane formulators, would be valuable.
It is apparent from the above there is an ongoing need for sustainable sources of polyester polyols which at the same time can help to both reduce waste streams, and provide further options for using under-utilized recycled polymer streams.